Validate not null constraint on projects.project_namespace_id column

In #354616 (closed), we added a not null constraint. It's still marked as invalid. You can run the following to reproduce:

gdk psql
\d+ projects

Output:
Check constraints:
    "check_1a6f946a8a" CHECK (organization_id IS NOT NULL)
    "check_fa75869cb1" CHECK (project_namespace_id IS NOT NULL) NOT VALID

Implementation guide

We need to validate it by following https://docs.gitlab.com/ee/development/database/not_null_constraints.html#not-null-constraints-on-large-tables:

1. Create a merge request using prepare_async_check_constraint_validation to validate the constraint async as shown in the above link.
2. Check postgres.ai and see if the constraint is valid using \d+ projects. If you're a community contributor you can ping @abdwdd for this step.
3. Use validate_not_null_constraint to validate it in a new merge request.